1. Field of the Invention
The present invention relates to an optical receiving device for use in optical free space transmission, and more particularly to an optical receiving device and an optical receiving method in which a portion of an optical signal is deflected for optical axis detection only when the optical axis is misaligned to thereby achieve a high S/N ratio of a received signal.
2. Description of the Background Art
Wireless transmission, which requires no wiring, has become popular as a data communications method between portable information terminals such as mobile telephones and PDAs. Particularly, wireless LANs using radio waves in the 2.4/5.2 Hz band as carriers are widespread. However, despite the usefulness thereof, there are also disadvantages caused by the wave transpiration, such as interchannel interference, decreased levels of security, and limitations in the transmission speed (on the order of 10 Mbps).
Therefore, attention has been drawn to optical free space transmission using infrared rays having various advantages over radio waves. Advantages in using infrared rays as carriers include the high level of security because of the straightness/blocking property of light, and the possibility for high-speed transmission utilizing the wideband property of an optical carrier. Note however that due to the straightness of an optical carrier, the transmitter and the receiver need to face each other to some degree. Thus, with an optical receiving device, the flexibility in installation is higher as the range of optical reception is wider.
Conventional means for expanding the range of optical reception of an optical receiving device include the optical means where the range of optical reception is expanded by improving the optical system such as the use of a lens having a high numerical aperture, and the mechanical means where the optical axis of the optical signal outputted from the optical transmitting device is detected and the optical axis is adjusted by moving the optical receiving device itself, or a lens, a mirror, an optical receiving element, etc. With the latter mechanical means, the light receiving angle of the optical system is not as wide as that with the former optical means, but it is possible to receive only the optical signal in the vicinity of the optical axis by performing the optical axis adjustment, whereby the influence of unnecessary light can be reduced and the S/N of the received signal can be increased as compared with the former optical means.
A conventional optical receiving device using the mechanical means is described in Japanese Laid-Open Patent Publication No. 5-133716 (hereinafter referred to as “Patent Document 1”), for example. FIG. 11 is a block diagram showing a configuration of a conventional optical receiving device described in Patent Document 1. In FIG. 11, the conventional optical receiving device includes a total reflection mirror 900, a deflecting mirror 910, a signal light receiving element 920, an optical axis detection light receiving element 930, the condensing lenses 901 and 902, an optical axis detection circuit section 940, and an optical axis adjustment section 950.
The optical signal reaching the optical receiving device is reflected by the total reflection mirror 900 toward the deflecting mirror 910, and a portion of the optical signal is reflected by the deflecting mirror 910 toward the condensing lens 901, after which the portion is condensed through the condensing lens 901 and coupled to the optical axis detection light receiving element 930. The optical signal passing through the deflecting mirror 910 is condensed through the condensing lens 902 and coupled to the signal light receiving element 920.
FIG. 12 shows an exemplary general configuration of the optical axis detection light receiving element 930 used in the conventional optical receiving device. As shown in FIG. 12, the optical axis detection light receiving element 930 is a light receiving element whose light receiving surface is divided into a region 931, a region 932, a region 933 and a region 934, and is capable of outputting an electric signal according to the proportion among portions of a beam spot 935 coupled to the regions 931 to 934. The optical axis detection circuit section 940 detects an optical axis misalignment based on the electric signal outputted from the optical axis detection light receiving element 930 to thereby output an optical axis detection signal to the optical axis adjustment section 950. The optical axis adjustment section 950 adjusts the angle of the total reflection mirror 900 based on the optical axis detection signal.
However, with the conventional optical receiving device, a portion of the received optical signal is deflected by the deflecting mirror 910. Therefore, a portion of the optical signal is always deflected for the purpose of optical axis detection even where there is no optical axis misalignment, whereby the power of the optical signal coupled to the signal light receiving element 920 is reduced and the S/N of the received signal deteriorates.